1. Field of the Invention
The present invention relates generally to a method and system for cooling strip, such as steel strip and so forth. More specifically, the invention relates to a novel and useful laminar flow cooling system for establishing laminar flow of cooling fluid for cooling strips with substantially uniform cooling rate over the over all width of the strips. Further particularly, the invention relates to a laminar flow cooling system which can adjust the of flow rate of a cooling fluid as a cooling medium for adjusting cooling efficiency.
2. Description of the Background Art
Laminar flow cooling systems are employed in hot strip mill lines for cooling steel strip, for example. Such a cooling system is arranged between a finishing mill and a take-up roll for cooling strip fed along a run-out table. In such laminar flow cooling system, water is generally used as the cooling medium and discharged toward the strip in a form of a plurality of bars-form laminar flow aligned in a direction of the width of the strip, or in a form of slit laminar flow extending in the direction of the width of the strip so as to cover the overall width of the strip. Such laminar flow cooling systems have higher cooling efficiency than a spray-cooling system, in which high pressure water is sprayed toward the strip; for the former generates higher heat transfer coefficient than the latter. Therefore, such laminar flow cooling systems are known to allow higher speed production of steel strip in hot strip mill lines. Furthermore, particularly in the case of the slit laminar flow of the cooling water, highly uniform temperature distribution in the width of the strip can be achieved because of uniform cooling efficiency over the overall width of the strip.
One type of the laminar flow cooling system is known as a "pipe-laminar flow cooling system". In this system, water-bar form of laminar flow is formed by pipe laminar flow nozzles. The other type of laminar flow cooling system is known as a "slit laminar flow cooling system". This system employs slit laminar flow nozzles for establishing the slit laminar flow of the cooling water. The pipe laminar flow cooling system has been disclosed in the Japanese Utility Model Second(examined) Publication (Jikko) Showa 56-41848, for example. On the other hand, slit laminar flow cooling system has been disclosed in the Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-77710 and the Japanese Utility Model First Publication (Jikkai) Showa 57-170812. In the known laminar flow cooling systems, it is well known that slit laminar flow cooling systems will have a cooling efficiency at the magnitude of about 1.5 times to 2 times higher than the pipe laminar flow cooling systems.
However, the slit laminar flow cooling system has the following drawbacks.
First of all, the slit laminar flow cooling systems are complicated in construction in comparison with that of the pipe laminar flow cooling system. Secondly and more importantly, the conventional slit laminar flow cooling system have a fixed cooling water flow area to limit the range of cooling water flow rate variation. Namely, when relatively low cooling efficiency is desired, it becomes difficult to sufficiently reduce the cooling water flow rate without causing breaking of the slit laminar flow. On the other hand, when substantially high cooling efficiency is required, the flow velocity of the cooling water becomes excessive to cause sprushing of the cooling water on the strip to lower the cooling efficiency. Therefore, it is well known that the slit laminar flow cooling system is only effective within a limited range of cooling efficiency. Furthermore, in order to form the slit laminar flow of the cooling water by means of the slit laminar flow nozzle, the slitted gap has to be narrow enough, e.g. about 20 to 30 mm. This can allow accumulation of foreign matter, such as fur. Accumulation of the foreign matter will cause variation of the cooling water path area and thus will cause variation of the cooling efficiency. Therefore, it is required for the conventional slit laminar flow nozzle to be regularly cleaned.
In order to allow a wider range adjustment of the cooling water flow rate in the laminar flow established by means of the slit laminar flow cooling system, there have been proposed improved slit laminar flow cooling systems with adjustable slit sizes. Such slit laminar flow cooling system have been disclosed in the Japanese Patent First Publication Showa 57-103728 and the Japanese Utility Model First Publication Showa 59-171761, for example. According to the disclosures of these publications, one of a pair of flow guide plates is movable with respect to the other flow guide plate in order to adjust the gap between the fluid guide plates to thereby adjusts the cooling water path area. Though such systems allow wider range adjustment of the cooling water flow amount and/or cooling water flow velocity, they require mechanisms for movably supporting the movable flow guide plates. This makes the structure of the cooling systems more complicated. Furthermore, such systems require relatively complicated and troublesome manual adjustment of the gaps between the flow guide plates.
There have also been proposed other type of laminar flow cooling systems which allow adjustment of the cooling water flow rate for varying cooling efficiency for controlling grain size of steel, material microstructure of the steel strip and so forth to control the quality of the strip. Such laminar flow cooling systems have been disclosed in the Japanese Patent First Publications Showa 51-28560, Showa 54-57414, Showa 55-88921 and Showa 59-50911, for example. In the disclosures of the Japanese Patent First Publications Showa 51-28560, Showa 54-57414 and cooling water supply lines for supplying cooling water to the laminar flow nozzles. On the other hand, in the disclosure of the Japanese Patent First Publication Showa 59-50911, the laminar flow cooling system is provided with a flow control valve in the cooling water supply line and a flow-blocking plate for interrupting the flow from the laminar flow nozzle for providing an ON or OFF control of water reaching the strip surface. These systems may allow some flow control for the cooling water according to the desired cooling efficiency. However, due to mechanical lag-time in the flow control valve and due to lag in variation of the cooling water flow rate in the cooling water supply lines, responsiveness to water amount control is not satisfactorily high. Furthermore, even by the latter mentioned system, as disclosed in the Japanese Patent First Publication Showa 59-50911, control of the cooling water flow is limited to ON or OFF. Therefore, although the flow rate of the cooling water is variable according to the disclosed system, control response is slow in all but the ON/OFF control functions. Also, variable flow rate adjustments can only be made through a relatively small range.